Topological memory using phase-change materials
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Introduction Hard disk drives (HDDs) and USB flash memories are the core components in computers that are used to store data. These memory devices are nonvolatile. The storage data in the world’s computers and servers continue to increase, consuming large amounts of energy to access the servers. Computing and analysis using big data especially require huge storage memories with high-speed access. However, present HDDs and flash USB have fatal issues related to access speed. “Storage-class memory (SCM)” has been proposed to overcome these issues.1 Although magnetoresistance random-access memory (MRAM),2 phase-change RAM (PC-RAM),3 resistance change RAM,4 and ferroelectric RAM5 have been explored thus far, since 2017, PC-RAM has been the leader with the launch of the device Optane by Intel and Micron Technology.6 The memory mechanism of PC-RAM is simple, relying on an amorphous to crystal phase transition in a Ge-Sb-Te chalcogenide alloy that produces a three orders of magnitude electric resistance change.7 The alloy was invented in the late 1980s by Panasonic,8,9 and it was first commercialized as a nonvolatile recording film in rewriteable optical discs (DVD and Blu-ray) in the 1990s. The Ge2Sb2Te5 composition shows the best performance for write-erase durability, large optical contrast (refractive index), and switching speed.10 Due to its excellent performance and success in optical memory, the alloy was soon applied to electric resistive memory because of its large resistance contrast between the amorphous and crystal states.
Development of interfacial phase-change memory One area of needed improvement in PC-RAM is the current relatively low efficiency of switching energy consumption compared with competitive devices. The recording and erasing mechanism in PC-RAM is simple. For the Ge-Sb-Te alloy, the difference in electric resistance values between the amorphous (high) and crystalline (low) states is large (three orders magnitude). The crystalline and amorphous states are called SET and RESET, respectively. The phase transitions from SET to RESET and from RESET to SET are carried out by injecting an electric current high and low for a certain length of time. In the transition from SET to RESET, the Ge-Sb-Te alloy needs to be heated to the melting point (>930 K), followed by a rapid quench to form an amorphous state, whereas the reverse transition needs to be heated to the crystallization temperature (∼430 K). Therefore, this RESET process consumes relatively high Joule heating energy.10 To reduce the energy consumption, several thermalmanagement technologies have been proposed, and adoption of a low thermal conductive protection layer adjacent to the memory cell has been implemented.11 From the first law of thermodynamics, the smaller the melting portion, the lower the Joule heating energy. Therefore, recently reported memory cell size has been designed to be less than a few tens of nanometers, as shown in Figure 1.12,13 An alternative solution to overcome the issue is to use the second law of thermodynamics—redu
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